Inspection of patterns on both semiconductor wafers and masks used in lithographic processing is extremely important to ensure successful device fabrication. Even minute irregularities in a pattern can result in a nonfunctioning device.
Optical inspection is commonly used for inspection of masks and semiconductor wafers. Optical inspection systems focus ultraviolet (UV) or deep ultraviolet (DUV) light onto the pattern surface and collect the transmitted or reflected light signals to form an image of the pattern. Shorter wavelength light sources allow resolution of smaller feature sizes than longer wavelength light sources. However, light sources of wavelengths below 200 nm that also provide the necessary power and repetition rate for use in an inspection system are not readily available. Therefore, the minimum resolvable feature size of such systems typically is limited to approximately 100 nm for defects. Some optical inspection using high numerical aperture (NA) optics and/or improved inspection algorithms.
Scanning electron beam inspection is another inspection technology. Scanning electron beam inspection systems focus electron beams from an electron gun and scan the beams across the surface of a pattern. An image is formed by collecting secondary electrons emitted from the surface. The use of reflected electrons (rather than photons) allows electron beam inspection systems to resolve significantly smaller pattern feature sizes than are resolved by optical inspection systems. For example, some scanning electron inspection systems resolve minimum pattern feature sizes on the order of 50 nm for primary features and defects. However, electron beam inspection systems generally are much slower than optical systems and, as a result, electron beam inspection typically is used in wafer sampling rather than in the 100% wafer inspection needed during wafer processing.